This invention relates to electroluminescent panels, and more particularly to flexible electroluminescent panels and methods of making the same.
In spite of progress in the manufacture of electroluminescent lamps and panels, there remains a need to improve the integrity of such panels, to increase the brightness of the panels over the service life, to increase the service life, to provide versatility in the displays, and to lower the manufacturing costs.
The effective brightness of a panel at a given voltage drive potential and frequency, and the ability of the panel to maintain such brightness over a long life, is of paramount concern. For example, it has been estimated that in 100,000 miles, an automotive instrument cluster will log about 2,200 hours. An electroluminescent panel in association with such an instrument cluster must provide a service life in excess of 2,000 hours and preferably substantially beyond. A further requirement is that the light output begin at an initially high level, and remain substantially constant both as to output and color balance throughout the panel's useful life, which may be defined as the length of time required for the luminance to decay to a value of 50% of original output.
The relatively high cost of manufacturing electroluminescent panels may, in part, be attributed to difficulties in manufacturing, and the inefficient use of relatively high cost materials, such as semi-precious metals and phosphor. For example, in many configurations, where less than the full surface of the panel is to be used, it has been a practice to mask the unused portion, which practice is wasteful both of materials and of power required to drive the panel. Panels are often designed in such a manner that they consume a disproportionate amount of power for the surface area utilized, thereby necessitating the use of a power supply which is in excess of the actual net requirements.
Since an electroluminescent lamp is made up of a plurality of operable materials for specific purposes, often such materials which are obtained from variable sources or have differing basic configurations. Thus the base materials, coatings, phosphors, resins, pigments, electrodes, and the like, are frequently combined without reference or thought to full compatibilty of materials from one layer to the next. Lack of compatibility can result in mechanical as well as chemical anomalies, and may manifest itself in surface wrinkling, or bending of thin panels, or may result in the physical separation of layers, or the lack of good moisture barrier qualities at the interfaces and edges. The resultant difficulties can result in a physically poor product as well as a product which has a high susceptibility to moisture damage or other environmental factors leading to a shortened life. Further, such incompatibility may limit the extent to which the panel may be electrically driven, may reduce the effective light output from the phosphors, or otherwise decrease the brightness of the panel.
U.S. Pat. No. 3,312,851 issued April 4, 1967 to Flowers et al describes the desirability of maintaining the dielectric layer as thin as practical to provide a steep electric gradient thereacross, and further describes the difficulty of superimposing one or more clear coats of the same resin as was used for the phosphor layer, over the phosphor layer, where cyanoethylated polyvinyl alcohol is used as the embedding resin for the phosphor, and where the clear coating applied from a solution of the same resin tended to redissolve the phosphor coating, resulting in the penetration of the phosphor layer with an accompanying disturbance of phosphor distribution, and light impairment.
Flowers et al addressed this problem by adding an organic compound which included (among others) 2,4-toluene diisocyanate to the previous resin to form a phosphor embedding material. In one example, additional films of clear resin (i.e., cyanoethylated polyvinyl alcohol and polyisothiocyanate or polyisocyanate) were cast on top of the phosphor layer. However, Flowers et al appear to have used the same resin only in the phosphor layer and dielectric layer, did not appreciate or directly address the compatibility or lack of compatibility of the adjacent polymer resins, and did not use subsequent resin layers to form an opaque pigment or to form a back electrode, and they did not use a polyester laminating resin.
A further difficulty resides in the conventional placement of the electric power leads at the margins or edges of the panel. Such lead placement makes more expensive or complicated the use of electroluminescent panels in installations where it would be advantageous to bring the power leads into the panel at a location remote from the edges. Internal lead placement has usually involved only the power lead to the back electrode, and there exists a need to make internal power lead connections to the transparent electrode.
Decorated or decorative electroluminescent panels have been made in which only portions of the entire panel areas are energized, to form a pattern of lighted areas on the panel. Commonly, such selective lighting or decoration has been achieved by suitably configuring a back metal electrode into the pattern desired, with individual power leads attached to the metal electrode segments as required. Such arrangements are shown in U.S. Pat. No. 3,133,221 issued May 12, 1964 to Knochel et al and U.S. Pat. No. 3,225,664 issued June 13, 1967 to Buck, Jr. et al. In the configurations shown in these patents, no attempt has been made to restrict either the areas of application of the phosphor or the areas, size or limits of the transparent electrode to conform to the pattern. Therefore, a substantial area of phosphor remains unused, and the unused area of the transparent electrode increases the likelihood of short circuits or accidental groundings. Commonly, such configured panel arrangements employ a solid metal back electrode which could be cut or stamped to the desired configuration.